Process for increasing the storage stability of biodiesel and the use of 2,4-di-tert-butylhydroxytoluene for increasing the storage stability of biodiesel

ABSTRACT

A process is described for increasing the storage stability of biodiesel by metering a liquid stock solution which comprises 15 to 60% by weight of 2,6-di-tert-butylhydroxytoluene dissolved in biodiesel into the biodiesel to be stabilized up to a concentration of 0.005 to 2% by weight of 2,6-di-tert-butylhydroxytoluene, based on the overall solution of biodiesel.

This application is a continuation of U.S. patent application Ser. No. 10/703,263 filed Nov. 7, 2003, incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process for increasing the storage stability of biodiesel and more specifically, the invention relates to the use of 2,4-di-tert-butylhydroxytoluene (referred to hereinbelow as BHT) for increasing the storage stability of biodiesel.

2. Brief Description of the Invention

Biodiesel, which is currently being used to an increasing extent to replace petroleum diesel as a fuel for diesel engines, for combined heating and power stations, ships and boats, as well as motor vehicles. Biodiesel comprising of fatty acid alkyl esters, which are predominantly fatty acid methyl esters, is obtained by transesterification, in which vegetable oils such as rapeseed oil, soya oil, palm oils and other vegetable oils used in cooking oil or animal fats, are reacted with methanol in the presence of a catalyst (usually sodium hydroxide solution). Also obtained in addition to the fatty acid methyl esters which can be used as biodiesel is glycerol. This method of preparing biodiesel, known as the CD process, is described in numerous patent applications (DE-A 4 209 779, U.S. Pat. No. 5,354,878, EP-A-56 25 04). Since biodiesel has become ever more important as an alternative fuel for diesel engines, the production of biodiesel has also increased considerably in the last few years.

Compared to mineral fuels, however, biodesel is disadvantaged by its low storage stability. This is caused by the decomposition of the biodesel's high content of unsaturated fatty esters into short chain products, which reduces the energy value of this alternative fuel. The decomposition leads to precipitation, which is recognizable as cloudiness of the biodiesel. The resulting decomposition products are peroxides, aldehydes and short-chain free fatty acids which form soluble and insoluble polymers as precipitates. The acids tend to corrode injection systems, pumps and nozzles of diesel engines. The insoluble polymers, when crosslinked, can block the nozzles and lead to sparingly soluble deposits. This considerably restricts the functioning of the diesel engines. In addition, soluble and insoluble polymers which are formed by oxidative degradation from the overall decomposition products cause incomplete combustion with soot formation, which can in some cases lead to engine damage. Therefore, none of these decomposition products should be present in the biodiesel. Nonetheless, the increased use of biodiesel as an alternative fuel derived from renewable raw materials makes it absolutely necessary to increase the storage stability of biodiesel.

It is therefore an object of the present invention to distinctly increase the storage stability of biodiesel, i.e. of fatty acid alkyl esters.

SUMMARY OF THE INVENTION

It has now been found that 2,4-di-tert-butylhydroxytoluene (referred to hereinbelow as BHT), and alternatively known as 2,6-di-tert-butyl-p-cresol, distinctly increases the storage stability of biodiesel.

The present invention therefore provides a process for increasing the storage stability of biodiesel, in which a liquid stock solution, based on the stock solution, comprises 15 to 60% by weight of BHT dissolved in biodiesel is metered into the biodiesel to be stabilized up to a concentration of 0.005 to 2% by weight of BHT, based on the overall solution of biodiesel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of the results of Rancimate test showing addition of increasing amounts of BHT and the percent content of C18 acid methyl ester in the biodiesel correlating to the ppm of BHT added.

FIG. 2 shows that the difference in cloudiness before (left side) and after addition of BHT (right side).

DETAILED DESCRIPTION OF THE INVENTION

In the context of this invention, “biodiesel” refers to all saturated and unsaturated fatty acid alkyl esters, in particular fatty acid methyl esters, which can be used as biodiesel fuel, as customarily available under the name biodiesel as a fuel in automobiles for diesel engines in motor vehicles, combined heating and power plants, ships and boats, and other stationary diesel engines. Typically, the fatty acid methyl esters used as biodiesel are C₁₄-C₂₄ fatty acid methyl esters which may be pure or in a mixture. The biodiesels used in the process according to the invention may additionally comprise all customary additives as used, for example, to increase the winter stability of biodiesel. Typically, the biodiesels having storage stability improved by the process according to the invention are prepared by transesterification of methanol with vegetable oils such as rapeseed oil, soya oil, palm oil, used cooking oils and fats or animal fats. The biodiesel which is stabilized in accordance with the invention is preferably that which has been obtained from rapeseed oil or soya oil by the transesterification mentioned above.

In this context, “storage stability” means reduced cloudiness of the biodiesel, said cloudiness resulting from the reaction of decomposition products formed by oxidative processes in the biodiesel, giving rise to soluble and insoluble polymers which manifest in the form of precipitation.

The invention further provides a process for preparing a liquid stock solution for use in increasing the storage stability of biodiesel, in which liquefied, optionally distilled, BHT is metered at a temperature in the range from 70 to 120° C., preferably 90 to 120° C., with stirring into biodiesel up to a concentration of 15 to 60% by weight of BHT, based on the stock solution.

BHT is a solid at room temperature and can only be metered into the biodiesel at room temperature with great difficulty. The inventive stock solution comprising 15 to 60% by weight of BHT, preferably 20 to 40% by weight of BHT, is a highly concentrated solution of BHT in biodiesel which is liquid and meterable and can be very easily metered into the biodiesel to be stabilized. Even after a prolonged period, surprisingly, no precipitation of BHT out of this highly concentrated stock solution can be detected.

Typically, the stock solution mentioned can be metered into the biodiesel to be stabilized up to a concentration of 0.005% by weight to 2% by weight, preferably 0.1 to 1% by weight, based on the overall solution of biodiesel. It is also possible to add higher concentrations of BHT to the biodiesel. The greatest stability effects are observed up to a concentration of 2% by weight. In comparison to unstabilized biodiesel, the inventively stabilized biodiesel has considerably improved storage stability, i.e., no undesired precipitation of insoluble polymers generated by oxidative degradation is detected in the inventively stabilized biodiesel. In addition, it has been found that BHT advantageously leads to an increase in the solidification point of the biodiesel. In this context, “solidification point” refers to the temperature at which the biodiesel begins to crystallize out.

The invention further provides the use of BHT for increasing the storage stability of biodiesel by preventing the cloudiness of biodiesel as a result of the decomposition products formed by oxidation processes. The cloudiness manifests itself in the form of precipitation. In accordance with the invention, this precipitation can now be prevented from blocking engine nozzles or generating undesired deposits in the internal engine chamber (pistons, lines) by incomplete combustion, which can lead to engine damage.

The invention further provides the storage-stabilized biodiesel itself which contains 0.005 to 2% by weight of dissolved BHT.

The invention is further described by the following illustrative but non-limiting examples.

EXAMPLES Examples 1-5

Biodiesel (from rapeseed oil) was subjected to a Rancimate test admixed with increasing amounts of BHT (trade product from Bayer AG: Baynox®).

Performance of the Rancimate Test:

The Rancimate 679 (Metrohm) consists of a control section and a wet section. In the wet section, the samples are heated and blown through by air in the presence of copper. During the oxidative ageing, short-chain, volatile organic acids are formed and are introduced into a measuring cell filled with distilled water. The conductivity is measured and recorded there continuously.

The end of the ageing and oxidative stability is indicated by a steep increase in the conductivity. The time until the turning point is reached is referred to as the induction period and serves as a measure for the ageing stability.

In the examples which follow, all samples were subjected to the same experimental demands.

Time: 120 minutes at 70° C. while passing through 60 ml/h of air/hour.

Afterwards, the samples were investigated by GC analysis for their content of unsaturated fatty acid methyl esters. The following table shows the results:

Bio- diesel Oil mill Comp. Empty exam- Example Example Example Example Example Rape- ple 1 2 3 4 5 Fatty acid seed oil BHT BHT BHT BHT BHT BHT methyl methyl 0.0% 0.02% 0.04% 0.06% 0.08% 0.1% ester ester by wt. by wt. by wt. by wt. by wt. by wt. C16/ 0.2 0.2 0.2 0.3 0.2 0.3 1× double det. C18/ 21.6 0.4 0.9 2.3 3.7 5.5 11.3 2× double det. C18/ 67.4 43.7 50.8 58.0 60.5 62.7 64.9 1× double det. C22/ 0.2 1.3 1.6 1.7 1.8 1.7 1.7 1× double det. C24/ 0.3 0.3 0.2 0.2 0.2 0.2 1× double

FIG. 1 shows the results in graphical form.

The results show:

The higher the content of BHT in the sample was, the greater the proportion of polyunsaturated fatty acid methyl esters. The sample which had not been admixed with BHT exhibits strong degradation of unsaturated fatty acid methyl esters. BHT is able to limit the degradation of unsaturated fatty acid methyl esters in the biodiesel as a function of dose.

A 5 l flask having a narrow neck was charged with 2 l of biodiesel, and a second flask with the same amount, but admixed with 0.05% of BHT. The vessels were not sealed and left to stand at room temperature with occasional shaking (2-3 times per week). After approximately 6 weeks, the first cloudiness could be observed in the product which had not been admixed with BHT. After a further week, cloudiness as a result of insoluble polymers could be clearly seen.

In the experiment with the addition of BHT, there was no cloudiness whatsoever as a result of insoluble polymers even after 8 weeks.

Examples 6-8

To assess the oxidation stability of biodiesel (from rapeseed oil), the following measurement methods were carried out:

Measurement Methods:

Biodiesel was investigated using oxygen under pressure (10 bar) by the DSC method (Differential Scanning Calorimetry according to DIN No. 51007). To this end, pure biodiesel and biodiesel with increasing amounts of BHT were used in the test arrangement.

Determination of the heat flow to a sample, measured relative to a reference sample, both of which are subjected to a defined temperature program, is the basis of dynamic differential scanning calorimetry (DSC). It is possible by means of this method to determine specific heat, glass transition, melting and crystallization processes, thermal effects, purity, polymorphicity, chemical reactions and reaction kinetics. In most cases, a dynamic temperature programme is run through, i.e., the temperature range of interest is covered.

Comparative Example 2 and Examples 6 to 8

Beginning Bio- Heating of Energy diesel BHT [% Oxygen rate oxidation released [mg] by wt.]* [mg] [K/min] [° C.] [J/g] Comparative 100 — 10 1 59 490 Example 2 Example 6 100 0.1 10 1 97 510 Example 7 100 1.0 10 1 104 580 Example 8 100 5.0 10 1 104 430 *based on the amount of biodiesel used

Evaluation of Examples 6 to 8

When pure oxygen was added (approx. 10 bar) Comparative Example 2 (biodiesel without BHT) exhibited the beginning of the strongly exothermic oxidation reaction in the DSC from as low as approx. 60° C.

In Examples 1 to 3, the biodiesel was analyzed with different BHT additions with the addition of oxygen. These examples show that where as little as 0.1% BHT is added, the oxidation reaction does not set in until 97° C., although it proceeds with a distinctly higher heat production rate. Increasing the amount of BHT to 1% only achieves slight additional stabilization, i.e., the oxidation in this case does not set in until 104° C. A further increase in the amount of BHT to 5% does not cause any increase in the stability.

Example 9 Preparation of the Solution of Bht in Biodiesel

In a two litre stirred flask, 1 500 ml of biodiesel are stirred at room temperature. 300 g of BHT in liquid form are metered in from a dropping funnel heated with steam or water to 80 to 90° C. within 10 minutes, in such a way that the BHT goes immediately into solution.

Afterwards, the mixture is cooled to room temperature and transferred through a filter and into a 21.5 l metal canister.

The 20 g/l solution exhibits no cloudiness or precipitation whatsoever even after storage at 0° C. for two weeks.

Example 10 Prevention of Precipitation

2 l of the same sample of biodiesel were each fed into two different large-volume

5 l vessels so that the vessels were each only half-full and the biodiesel had as large as possible a surface area.

In the vessel P5-0.0, no BHT was added, and in the vessel P5-0.05, 500 ppm of BHT were added.

After 30 days at room temperature in the closed vessel, distinct cloudiness is observed in the P5-0.0 vessel without BHT addition, said cloudiness being caused by crosslinked insoluble polymers in the biodiesel. In contrast, the biodiesel in the P5-0.05 vessel having 500 ppm of BHT remained completely clear and transparent, and no insoluble polymers whatsoever precipitated out.

FIG. 2 illustrates the result.

Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims. 

1. Process for increasing the storage stability of biodiesel, comprising metering a liquid stock solution which, based on the stock solution, comprises 15 to 60% by weight of 2,6-di-tert-butyl-hydroxytoluene dissolved in biodiesel into the biodiesel to be stabilized, up to a concentration of 0.005 to 2% by weight of 2,6-di-tert-butyl-hydroxytoluene, based on the overall solution of biodiesel.
 2. Process according to claim 1, characterized in that the stock solution is metered into the biodiesel up to a concentration of 0.1 to 1% by weight of 2,6-di-tert-butyl-hydroxytoluene, based on the overall solution of biodiesel.
 3. Process according to claim 1, characterized in that the biodiesel is a biodiesel which has been obtained by transesterification with methanol from rapeseed oil, soya oil, palm oil, used cooking oil or animal fat.
 4. Liquid stock solution for use in a process for increasing the storage stability of biodiesel, comprising, based on the overall stock solution, 15 to 60% by weight of 2,6-di-tert-butyl-hydroxytoluene dissolved in biodiesel.
 5. Storage-stabilized biodiesel, comprising 0.005 to 2% by weight of dissolved 2,6-di-tert-butyl-hydroxytoluene. 